Fiberglass yarns, woven into a weave pattern, typically provide mechanical strength for a printed circuit board. The weave pattern is impregnated with an epoxy resin, which provides electrical insulation and adhesion for the printed circuit board.
The fiberglass yarns and the epoxy resin typically have different physical properties pertaining to electrical conductivity. For example, the fiberglass yarns and the epoxy resin can have different values of relative permittivity (Dk) and different values of loss tangent (also known as dissipation factor) (Df). These differing physical property values can lead to different speeds of signal propagation in the printed circuit board, depending on which materials are adjacent to a signal trace. For example, a signal in a trace routed mostly over fiberglass can travel slower than a signal in a trace routed mostly over resin.
The variation in signal speed as a function of adjacent material can lead to a problem known as the fiber weave effect. The fiber weave effect can desynchronize signals that are sent over two transmission lines of the same length if the transmission lines are positioned (on average) near different materials. For a printed circuit board that uses high-speed differential signal pairs, the fiber weave effect can desynchronize the two signals in the signal pair, and can introduce unwanted noise in determining when voltage changes occur in the differential signal pair.
There have been numerous attempts to mitigate the fiber weave effect. Some have the disadvantage of increasing the surface area or board footprint for a particular circuit. Some have the disadvantage of wasting particular areas on the printed circuit board. Some have the disadvantage of increasing cost, due to additional material processing steps or fabrication processing steps. Some have the disadvantage of limiting a bandwidth or preventing a scaling of the circuit. None are satisfactory.
Corresponding reference characters indicate corresponding parts throughout the several views. Elements in the drawings are not necessarily drawn to scale. The configurations shown in the drawings are merely examples, and should not be construed as limiting the scope of the inventive subject matter in any manner.